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I have been running a hot end of my own design for a while now, but using an external power supply for my hot end fan, and happened to pick a print fan which has lived at the supplied 19v.

I rev'd up my design to be smaller, lighter, stiffer, and much shorter as well. Now seems like a good time to clean up and integrate the electronics.

I am pretty sure that my base electronics are 1.5.7, and I have added the official heated bed kit. I believe this gives me access to constant on 19v and 24v sources. What I am unclear on, is whether I have a 5v extruder fan connection that can be controlled via firmware (on when the hot end is above a set point).

For the hot end fan, I see 3 potential options:

Use the 5v lead if I have it. Could I use this to step the voltage up to 12v? DC Step up

Piggyback, in parallel, on the 12v leads that power the control board fan and hope not to overload it with an additional ~0.1 amps

Grab the always on 19v or 24v leads, and run a step down converter: DC Step down

Any thoughts on this?

For dual print fans:

I will be running 12v 30mm fans (same as above @ ~0.1 amp). I can either wire them in parallel and hope they don't burn up. But I think using the step-down converter above on the normal fan output would be a better solution. My biggest question on that, is if wiring that converter inline with the fan output would cause issues in the PWM fan signal.

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The dual print fans should run via the existing PWM circuit. I went ahead and just ran a couple cheap 12v 30mm axial fans, in parallel, on that circuit. So far they have survived. I'm not sure what the worst case is if they fail. It would suck to burn out the transistor if they fail as a short. Not sure if fans ever really do that.

I'm running a complete external power supply for the hot end fan right now. I would like to clean that up and have a pretty good path to take. I can solder in my voltage step down board, into the UM supply circuit and step down to 12v so that fan has a higher chance of success.

I'm just curious how those voltage regulator board would handle a PWM signal (for the print fans).

It has a switching frequency of about 1 Mhz, while the board has a PWM frequency of < 38 kHz. That would be ok.
And the IC on that step down board has a startup / shutdown time of "a few ms" and that kills it, it can't keep pace with any PWM that's much higher than 100 Hz.

I would not feel comfortable using a step-down powered by a PWM signal.

What you could do is create a small circuit powered by the step-down (19v > 12V, taking the 19V from somewhere else on the board) using the original fan PWM as input to switch a MOSFET that pulses the 12V output of the step-down. That's a really small circuit out of a MOSFET and a resistor. For 0.1A you could even use a normal small npn transistor (BC337 or something).

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